I don't think it really matters what new cars enter the category. Although seeing Merc, BMW, Chrysler and Audi shaped cars would be cool, the cars are all going to be running the same gear anyway.
As long as the racing is still close, there's lots of real passing (pit lane passes don't count) and some cars get bent every now and then, I'll be happy. The racing is generally pretty good. It just needs to be as far from F1 as possible.
If you want a category with real variety check out the Aust GT championship. Audi, Ascari, Porsche, Lamborghini, Nissan, Lotus, Dodge, Ferrari, Mosler and more. All they need is for channel 7 to broadcast it properly and they'd be on a winner. Some brand name drivers probably wouldn't hurt either, although seeing Tony Quinn crash two Mosler's in one weekend was pretty entertaining. How much money is in pet food anyway?